Turbo fan powered aircraft are required to cruise for considerable distances at high speed. Thus, propulsion system efficiency of the aircraft must be maximized. On the other hand, the propulsion system of the aircraft must be capable of ingesting foreign objects without engine damage. The problem of foreign object ingestion has been solved in the past by merely increasing the strength of the engine components exposed to impact damage. However, strength can be equated with weight, which, in turn, compromises performance of the aircraft. Reconciliation of such seemingly divergent performance and safety requirements requires careful integration of the aircraft's propulsion system with airframe aerodynamics.
The basic model of air flow past an aircraft fuselage assumes that a relatively thin region adjacent the aircraft's fuselage termed the "boundary layer" exists in several states, namely, laminar, turbulent, wake and an external stream. In the laminar state, flow is stratified. Farther aft, laminar flow transforms to a turbulent state which is eddying in character. At the aft end of the aircraft turbulent flow transforms into a wake wherein the direction of flow may actually reverse. The external stream is outboard of each of the aforesaid boundary layer states.
While both laminar and turbulent airflow along the fuselage of an aircraft tend to follow the contour of the fuselage, relatively heavy foreign objects, for example, birds, tend to flow directly rearwardly of the aircraft fuselage in the external stream due to inertial forces if the object is forced into the external stream.